Multivibrator



Patented Apr. 27, 1948 MULTIVIBRATOR Thomas L. Dmond, Rutherford, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 13, 1945, Serial No. 582,457

6 Claims. (Cl. Z50-36) This invention relates to electric oscillators of the multivibrator or relaxation oscillator type, and particularly to means for controlling the form of the saw-tooth wave voltage generated and produced by such oscillator apparatus.

One of the objects of this invention is to control the form of the voltage wave produced by an oscillator of the multivibrator type.

Another object of this invention is to provide a linear or nearly linear form for a saw-tooth wave voltage generated by an oscillator of the multivibrator type.

Another object of this invention is to reduce the effect upon the form of the saw-tooth wave of extraneous voltages that may be induced by stray elds in long lead lines associated with an oscillator of the multivibrator type.

Electric oscillators of the multivibrator type may be utilized to generate waves of the sawtooth form and of other forms. Such waves may be utilized for controlling the sweep of a cathoderay tube for example, and for other purposes. When -a saw-tooth Wave is used for controlling the timing action in a. radio system for example, it is often desirable to have a linear form of wave for better control of the timingaction. 'I'he linear form of the saw-tooth wave has a rate of change oi voltage which is uniform with time, and accordingly, is useful in obtaining uniform time divisions in apparatus to be controlled, as in the sweep of an oscilloscope for example.

kIn accordance with this invention, the sawtooth Wave generated by a multivibrator may be rendered of linear or nearly linear form and also relatively free from the eifects of extraneous voltages that may be induced in the lead lines of the output circuit by stray fields. For this purpose, a suitable negative bias potential may be utilized in connection with the condenser discharge circuit of the multivibrator in order to obtain a nearly linear discharge characteristic for the form of the saw-tooth Wave voltage generated during the discharge of the condenser through its associated resistance. Also, an additional condenser or condensers connected in parallel circuit relation with the condenser of the discharge circuit and comprising a part of the capacitance thereof may be provided at the far end of the lead line for providing a low impedance to thereby reduce the effect of extraneous voltages that may be induced in the lead line by nearby stray fields. These two features may be utilized in combination or separately, the

serve such linear wave form in its transmission to the load or work circuit.

For a clearer understandingV of the nature of this invention and the additional advantages, features and objects thereof, reference is made to the following description taken in connection with the accompanying drawing, in rwhich like reference characters represent like or similar parts and in which: i

Fig. 1 is'a circuitV diagram of a multivibrator in accordance with this invention;

Fig. 2 is a graph illustrating the linear form of the voltage wave generated by the discharge of the RC circuit of Fig. 1; and

Fig. 3 is a graph illustrating the complete sawtooth voltage wave generated by the multivibrator of Fig. 1. f

Referring tothe drawing, Fig. 1 is a circuit Vdiagram of a self-oscillatingmultivibrator coinprising two amplier type vacuum tubes vVI and V2 each having a cathode 3, a grid or control electrode 4 and ananode or plate electrode 5. The two ampliJ'ier tubes Vl and V2 are resistance-capacitance coupled, the output of one tube being fed to the input` of the other, and the frequency of oscillation being determined by the time constants'of the resistances and capacitances of the circuit. The plate electrode 5 of the second tube V2 is connected vviththel grid electrode 4 of the first tube Vl` bymeans of a capacitance C4, a resistance R2 and a resistance R4. The plate electrode 5 of the first tube VI is connected with 'the grid electrode 4 ofthe second tube V2 by means of a condenser C5 and agrid resistance R42. Thegrid 4 of the tube VI is connected to ground I through the resistance R4 and the series-connected resistances R3 and R41. If desired, aterminal =2 Amay be `connected with a pointV intermediatejthe resistances R3 and R41 for the purpose of introducing 'a synchronizing voltage into the grid circuit 4 of the tube Vl. 'I'he `grid electrode 4, of the second tube V2 is connected with ground! through the grid resistance R42 and aresistance R1. The cathode 3 of the iirsttube Vgl may be connected toground I through a resistance R5, Any suitable lament and power supply source (not shown) may be used for' heating the cathodes 3 of the tubes VI and V2 and also o f the output circuit tubes V3 and V4. A. suitable power `supply source IU and associated anode resistances R6 and RI'3 and R44 may be provided Vfor .maintaining the anodes 5 of the tubes Vl and V2 at a positive The cathode 3 of the second tube V2 is connected through a capacitor or condenser C9 to the ground I, and through a variable resistance RII to a tap point II on a voltage divider comprising the resistances R8 and R9. A source of negative bias potential I2 connected with the resistance R8 and the ground `I supplies a potential -a't the tap II and also at point A 'in Fig. 1, which is negative with respect to the ground I. far end. Accordingly, current flowing through the Voltage divider R8 and R9 produces ,at the tap II, a negative potential with respect to the ground I.

the tube V2. The saw-tooth wave generated appears on the lead wire I4 connected with the cathode 3 of the tube V2. A condenser C9 may be connected with and intermediate the plate resistances RI3 and R44 in order to eliminate the effect of ripple from the .plate "power supply source I upon the saw-tooth wave generated at the output lead wire I4.

The saw-tooth wave voltage 2from'the lead wire i4 may be suppliedover `one or more relatively long lead wires `Iii and II toloador work circuits Vrepresented by the tubes V3-and V4 respectively, which may be for example electronic relays, or amplifiers. At the far ends of the long leadlines I6 and II and adjacent the tubes V3 'and V4, condensers CB 'andCIIlrespectively/may be connected in parallel -circuit'relation with the condenser CG and form partv of thetotal Vcapacitance of the RC circuit. ResistancesRI'lI Iand'RIi may be connected inparallel circuit relation withthe condensers C8 and `CII) respectively. "The condensers C'IA and C'IB are large blocking condensers interposedin'thelong leads A'I6 Vand II, respectively. The RC circuitcapacitance, which may comprisei'the single condenserCSor. in addition other smaller condensers C8 and "CID connected vin .parallel".circuitrelation therewith, is included in .the .cathode circuit? ofthe tube`V2 to develop the saw-tooth wave. "The large resistances RI"4 vand `RI5 associated' with vthe condensers C8 and'CID, respectively maybe considered asbeing connected inparallel circuit relation withthe principal resistance RII.

LIn a particular example for .a multivibrator of about `2`6 cycles ,per secOndrequenCy, the .values ofthe resistances andthe capacitancesgchosen for-use in fthe Jcircuit dfFg'; '1 were substantially as follows; Rh-447,000 ohms, .TRL-510,000 ohms, R`4=470,000 ohms, "R5-Z700 ohms, R$="51,000

The resistance R9 is grounded at its The condenser Cldischargesthrough the resistances R9 and RII, and is chargedfrom conventional vacuum tubes, and the biasvoltage .and-theresistancesRZ, R3 and R41.

of the voltage across the condenser C6. This condition is keeping the tube V2 blocked. The condenser C6 slowly discharges through the resistance RII approaching a Voltage of -70 volts, the voltage between the resistances R8 and R9 with no current flow in the resistance RII on its top plate. However, os soon as the condenser C6 has discharged enough toflose about 18 volts in the present example, the grid bias potential becomes low enough that the tube V2 starts conducting, thus causing its plate Voltage to change in a nega- .tve'direction This negative change in voltage is `passed Vto the grid 4 of the tube VI by the RC coupling network vconsisting of the condenser C4 The amount of voltage change is reduced because the resistances R2, R3 and R41 form a voltage divider as thegrid.4 of the tube VI is driven in a negative direction in potential, the current in the plate circuit of the tube VI is reduced and the plate voltage changes in a, positive direction. This change is-passed to the grid 4 of the tube V2 by the RC coupling circuit comprising the resistance R'I-and the condenser C5. This tends to increase the current in the plate circuit 5 of the tube V2, thus causing the plate 5 to become still more negative. Because of this regenerative action the current in the Vplate circuit of the tube V2 rises very quickly to a maximum value determined by the tube'characteristics. Most ofthe plate current consists of the charging current of the condenser CB although there is a small amount of current through the resistanceRI I.

The charging ofthe condenser 'CtV will continue until'the plate current ofthe vtube V2 is `blocked by the following uneans. When the tube 'V2 started conducting, the grid A4 of the tube VI was driven to cut-off by 'theRC couplingcircuit consisting ofthe resistances R2, R3, R47 and the condenser C4. Now, even though thereisa nearly constant'voltage at the plate "5 of the tube V2 during the vcharging of the condenser C6, there will be a changing voltage across .the condenser C4 because the plate of the condenser C4 Vwhich is connected to the resistance 'R2 will tend to assume ground potential. AAs soon as this plate ofthe condenser C4 approaches suiciently near to the ground potential, the grid 4. of the tube VI will no longer be at cut-.off potential and the tube VI will vstart conducting. Its .plate ,5 lwill vbecome more negative, the grid 4.ofthe tube V2 `will become more negative, the .platecurrent in the tubeVZ will be reduced, theplate voltage of the tube`V2 will become morepositive .and its plate still more negative. Hence the` action will quick- 'ly build up until the platecurrent of the tube V2 is cut off. vWhile the tube`V2is cut oif,..the plate of the condenser 'C4 which is ,connected tothe resistance R2 reaches ground potential,.the.tube VI remains conducting with a grid .biasequal to the voltage across the resistancelR. andthe grid 4 .of `the tubeVZ .attains ground `potential. yThe .timeconstant of the .condenser C5 and-.theresistance'RI is such that this network acts as a. good low`frequency coupling circuit. That is, thevoltage change .across the condenser.C5.dur-ing the time the tube'VZ is cutoff is small. .With the Vtube'VZ cut off, the action described aboverepeats.

The tube V2 is blocked during v.most ofthe multivibrator. cycle. .But duringthe .short .period that the tube .V2..is conducting,-most-of the l. plateo cathode .currentthereof ilows throughrthe .ca-

ance R1, and the cathode 3 is positive withref spect to ground I and the grid 4 by the amount Lpac'itor CI-. to` ground` I,v therebyf rapidly charging vth'e condenser C6. -When theA tube i-V2- isY blocked,

the condenser C6 discharges rather slowly through the resistances R9 and RII due to the relatively high resistance of the resistor RI I.l The low voltage point A of the resistance RII in Fig. 1 is carried to about 70` volts, in the example given, by means of the voltage divider R8 and R9 supplied from the 250 volt supply'source I2. With this arrangement the condenser C6 loses only ya relatively small part of the total charge during the 'discharge operation, but provides a full or'substantial voltage drop for the saw-tooth wave at the lead line I4, the wave being practically linear as illustrated by the downwardly sloping prtion B of the curves in Figs. 2 and 3. l

In the conventional form of multivibratorcircuit, the point labeled A inthe cathode lead of the tube V2 in Fig. 1 would be connected to ground I. Under th'at condition, the condenser C6 will discharge from a voltage which may be, for eX- ample, about 28 volts when the tube V2 is blocked, to about volts for example, at which time the tube V2 starts conducting. This gives a swing of some 18 volts which represents or 64 per cent of the starting voltage in RC circuit of the multivibrator. When the circuit is connected as shown in Fig. 1, the starting voltage on the cathode 3 of the tube V2 is still the same 28 volts and it still drops to the same 10 volts before the tube V2 starts to conduct. However, when the circuit is connected as shownin Fig, 1 where, instead of being grounded at the point A, a Voltage of -70 volts for example is applied at the point A from the voltage source I2 through th'e resistance R8, there results a total starting voltage in the RC circuit of 70|28=98 volts, which decreases by 18 volts in the example assumed. 'Ihis results in a swing of 18 volts in 98 volts which represents or 18 per cent of the starting voltage in the RC circuit of the multivibrator. The curve of voltage versus time of the condenser C6 discharging through' the resist-ance RII is nearly linear forabout the first per cent of its final or total voltage change, but is not nearly linear if some larger percentage voltage change is involved, as illustrated by the curves A and B in Fig. 2. Accordingly, by operating the discharge of the condenser CB on only a relatively small part, such as less than about 20 per cent for example, of its total charge, a substantially linear form of wave is generated, as illustrated by the curve B in Figs. 2 and 3.

Fig. 2 is a graph illustrating typical curves of `voltage versus time for a condenser such as the condenser C6 discharging through a resistance such as the resistances R9 and RII in Fig. 1. In Fig. 2, the ordinates may represent the values of voltage on the cathode 3 of the tube V2 Where the value of +28 volts illustrates the maximum voltage on the cathode 3 of the tube V2, where the value of -l-10 volts represents the cut-ofi voltage of the tube V2, where the value of 0 volt represents the potential at ground I, and where the value of -70 volts represents the voltage applied at point A of Fig. 1 by the tap point II of the voltage divider R8, R9 and the source I2.

The curveA of Fig. 2 represents the non-linear form of voltage characteristic of the condenser C6 of Fig. 1 discharging through' the rsistance RII of Fig. 1 that is obtained if and when the circuit is grounded at the point Aof Fig. 1 and the resistance RII is readjusted to obtain the same cycle time. The curve B of Fig. 2 represents the nearly linear form of voltage characteristic that may be obtained from the condenser C6 discharging through the resistances R9 and RII of Fig. 1 when a negative bias voltage is yapplied at point A of Fig. 1 of suitable value to keep the discharge voltage change of the condenser C6 within about 20 per cent of its total voltage charge.

The data for the curves A and B of Fig. 2 were obtained using the well known formula for the voltage across a condenser discharging through a resistance:

Fig. 3 is a graph illustratinga typical form for the saw-tooth wave generated by the multivibrator circuit of Fig. 1. The portion of the curve. labeled B in Fig. 3 corresponds to the curve B in.y Fig. 2 and represents the form of the wave gen-Y erated during the discharge of the condenser C6 through the resistors R9 and RII in Fig, 1.. 'I'he tube V2 is blocked during this part which is: the greater part of the time cycle; but during th'eremainder or lesser part of the time cycle, thetube V2 is conducting and charges the condenser C6, as indicated by the upwardly sloping part A of the saw-tooth wave curve in Fig. 3.

It will be noted that the cathode circuit 3 of the tube V2 of the multivibrator in Fig. l is provided with a condenser C6 and parallel-connected resistances R9 and RII which taken together form the RC circuit, and that one feature of this invention involves using a voltage divider for providing a negative bias potential in connection with the resistance and condenser of the RC discharge circuit in order to obtain a linear or nearly linear discharge voltage characteristic for the form of the saw-tooth wave voltage generated. This substantially linear voltage characteristic with time is illustrated by the downwardly sloping curve B in Figs. 2 and 3. Also, it will be noted that the substantially linear form of the saw-tooth wave as a whole may be preserved from interference in transmission over the associated lead line by means of another feature of this invention which involves the use of an additional but smaller condenser CB or condensers C8 and CII] connected in parallel circuit relation with the condenser CB and comprising a part of the total capacitance of the principal RC circuit, but disposed at the far end of the output lead line I6 or I1. This arrangement provides a low impedance and thereby reduces the eiect of any extraneous voltages that may be induced in the long lead line I6 or I 'I by stray elds. v

. Although this invention has been described and illustrated in relation to specic arrangements, it is to be understood that it is capable yof @limitation in other organizations and is therefore not to be limited to the particular .emloodimen-ts disclosed.

What is claimed is:

1. An oscillator for producing a saw-tooth Wave Vform comprising a pair of electron dis,- charge tubes each including a cathode, a control electrode and an anode, means including anode ,resistances for maintaining lsaid anodes positive with respect to said cathodes, means including ,capacitances vfor connecting the anode of each of said Vtubes with the control electrode of the other of said tubes, means including resistances yfor connecting said control electrodes with one of said cathodes, means including another resistance for connecting the other of said cath,- odes with a source of direct current bias pfotcntial which is negative with respect to said one of said cathodes, and means including a condenser connected across said another cathode resistance, the value of said negative bias potential being related to a partial capacitance discharge value of said condenser and comprising means for obtaining a nearly linear discharge characteristic for the :form of said saw-tooth wave when said condenser discharges through said another resistance.

2. A generator for producing a saw-tooth wave comprisingv a pair of electron discharge devices each including a cathode, a control electrode and an anode, mea-ns including anode resistances -for maintaining said lanodes posi-tive with respect to ground, means including )a series-connected capacitance and resistance for connecting the anode of each of said discharge devices with the control electrode of .the other of said discharge devices, Xa. pair of resistances, means for connecting one end of each of said `pair of resistances to said control electrodes and the other ends of said pair of resistances to said ground, means including a series resist-ance tor connecting said cathode of one Aof said discharge devices with said ground, means for connecting said cathode of said other of said -pair of said kdischarge devices with said ground including a condenser .connected between said last-mentioned cathode and said ground and including a cathode resist .ance connected at one of its ends with said last.- y-menltiohed cathode and at its kother end with a grounded voltage divider at a point thereon where said Voltage :is negative with respect to said ground, means for suppl-ying said .point on Said voltage divider with said .negative voltage :of a value lthat keeps the discharge voltage across said r,condenser within substantially 2 0 per cent l,

f .the total voltage charge applied ,to said -conidenser yvv-here'oy a substantially linear .wave iform is -genera ted by said condenser discharging Ktl'irough said llast-mentioned cathode resistance.

3. A system in vaccordance with claim 2 and apparatus connected Ytherewith QOIIlDlTSiilg :.a.S 1W ,tooth wave output circuit connected with .said last-menticnedl ,cathode and -said ground, s aid output ,circuit comprising Va ,pair vof parallelconnected Vcircuits having relatively long ,leads whereby .extraneous voltages :may V,be ,undcsirahly iinduced iby stray hel-ds, each of vsaid parallel- -connected -fcircui-ts including `a series blocking condenser, 1a shunt ,resistance and ashunt ccn- Adenser, each of said ,shunt :condensers forming apart vof the capacitance of said condenser con nected with said last-mentionedcathode randeach or said shunt Acondensers heing made of values .to :provide Aa relatively low impedance .at each of fthe remote yends of .said leads whereby the ertect ,oisaid induced extraneous voltages on said saw-tooth wave in said leads is reduced 4, A system in accordance with claim 2 and apparati-1s connected therewith comprising `a sawtooth Wave output circuit connected with said last-mentioned cathode and said ground, said output circuit comprising a pair of parallel-connected circuits having Vrelatively long leads whereby extraneous voltages may be undesirably induced therein by stray fields, each of said parallel-connected circuits including a series-blocking condenser and a shunt condenser, each of said shunt condensers forming a part of the capacitance of said condenser connected with` said last-mentioned cathode and each of said shunt condensers being made of val-ues to provide a relatively low impedance at each of the `remote ends of said leads whereby the eiect of said induced extraneous voltages on said saw-tooth wave in said leads is reduced.

5- A generator for producing a Vsaw-.tooth wave comprising a pair'of elect-ron discharge devices each including a cathode, a control electrode and an anode, means including anode resistances for maintaining said anodes positive with respect to ground, means including a series-connected impedance for connecting the anode of each of said discharge devices with the control electrode of the .other of said disehargedevices, a pair of resistances, means for connecting one end of each of said pair of resistances to said control electrodes and the other ends of said pair of resistances to said ground, means including a series resistance for connecting said cathode of one of said discharge devices with said ground, ymeans y for connecting said cathode of said other of said pair of said discharge devices with said ground including a condenser connected between said last-mentioned cathode and said ground and including a cathode vresistance connected at one of its ends with said last-mentioned cathode and at its other end with a grounded voltage divider at a point thereon whe-re said voltage is negative with respect to said ground, means for supplying said point on said voltagedivider with said negative voltage-oi a value that keeps the discharge voltage across said condenser within substantially 20 per cent -,of the total voltage charge applied to said condenser whereby a `,s1-ilo,sta-ntially linear Wave form is ,generated by said condenser discharging Athrough 'said :last-mentioned cathode resistance, and output circuit means connected .with said last-mentioned cathode and said ground.

=6. A generator for producing ,a saw-.tooth wave comp-rising va pair .of electron discharge devices each including a cathode, Aa ,control .electrode ,and an anode, means including anode resistances for maintaining ,said anodes A:positive with respect to ground, fmeans `including la series-connected im- ;ledanceffor connecting fthe anode of .eachof said discharge devices with theicontrolelectrodeof the .other vof saijd .discharge devices,y ,a pair .of resistances, lmeans .-forconneoting one :end of each of `said paircf resistances .to said control electrodes :and the -other ,rends of said .pair .of resistances to said ground, means .including a series resistance -forlconnecting said `cathode of one of said discharge devices .with said .groundfmeans `forcen- Inecting lsaidcathode xof said other of -said Vpair ofsaiddischarge,devices with said ground'including a condenser connected between said lastmentioned cathode Aand said .ground :and :including acathode resistance connected at .one of its point on said voltage divider with said negative 5 voltage of a value that keeps the discharge voltage across said condenser substantially less than the total voltage charge applied to said condenser whereby a substantially linear wave form is generated by said condenser discharging through 10 said last-mentioned cathode resistance, and output circuit means connected with said last-mentioned cathode and said ground.

THOMAS L. DIMOND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,052,184 Lewis Aug. 25, 1936 2,355,287 Firestone Aug. 8, 1944 OTHER REFERENCES Ultra. High Frequency Techniques by Brainerd et al., page 185, July, 1942, D. Van Nostrand Co. (Copy in Division 51.) 

